Flapper valves are widely used in fluid conduits that transfer fluids between an oil well reservoir and a wellhead. Flapper valves are typically one-way valves that are hinged at one side of the conduit so that in an open configuration they are disposed generally parallel to the conduit, out of the bore, but can pivot over to a closed position in which they occlude the bore of the conduit and lie across its axis. In the closed position, flapper valves typically seal against an annular seat on the inner bore of the conduit, and fluid pressure behind the flapper typically keeps the flapper tightly closed against the seat, as long as the pressure differential across the flapper persists.
The flapper can move back into its original open position if the pressure differential across the seat is removed or reversed, allowing fluids to flow in one direction, but retaining pressure in the other.
Conventional flapper valves necessarily hold pressure in only one direction, and permit fluid transmission in the other.